Some enzymes of nucleoside metabolism of Escherichia coli.

We have recently presented a hypothesis of the mechanism by which the mutation rate of Escherichia coli is increased in the presence of methylxanthines (1). It was suggested that methylxanthines partially inhibit the synthesis of some nucleic acid intermediates, thus causing an increased probability for the utilization of a substitute compound in the synthesis of the specific polynucleotide that carries the genetic information. This hypothesis is consistent with two other findings: first, the complete absence of incorporation of radioactive methylpurines by growing cultures (2), and, second, the observed inhibition by methylpurines of the purine nucleoside phosphorylase present in extracts of this organism (1). In the work presented here, we have extended these enzymatic studies. A single preparation of an extract of E. wli was assayed under a variety of conditions for its ability to attack various nucleosides. By using a single preparation, we were able to make reproducible measurements without the complication of batch to batch variation in enzymatic activity which was noted previously. The effects of a number of purine and pyrimidine compounds on the various nucleoside-splitting activities were then measured by carrying out each reaction in a suitable buffer at the appropriate pH. From this study the existence of two additional types of enzymes, a hydrolase and transribosidases, was inferred. These activities were then directly demonstrated by separations achieved by starch block electrophoresis. The relationship of inhibition of enzyme activities to the hypothesis of mutagenesis as well as to other biological effects of these purines and pyrimidines is discussed.